MM is defined by the presence of clonal bone marrow plasma cells (≥10%) or biopsy-proven bony or extramedullary plasmacytoma and any one or more of the following myeloma defining events (Table 25.1) [42]:

Myeloma defining events Details
Evidence of end organ damage that can be attributed to the underlying plasma cell proliferative disorder

CRAB criteria:

Hypercalcaemia: serum calcium >0.25 mmol/L (>1 mg/dL) higher than the upper limit of normal or >2.75 mmol/L (>11 mg/dL)

Renal insufficiency: creatinine clearance <40 mL per min or serum creatinine >177 μmol/L (>2 mg/dL)

Anaemia: haemoglobin value of >2 g/dL below the lower limit of normal, or a haemoglobin value <100 g/L

Bone lesions: one or more osteolytic lesions on skeletal radiography, CT, or PET-CT

Any one or more of the following biomarkers of malignancy

▪ Clonal bone marrow plasma cell percentage ≥60%

▪ An involved/uninvolved sFLC ratio ≥100*

▪ >1 focal lesion on MRI studies

Table 25.1. Myeloma defining events [42]. *These values are based on the Freelite assay. The involved FLC must be ≥100 mg/L.